Stroker Ace: Subaru's New FB Flat-4 Engine

Slightly more midrange torque, sweeter air out the tailpipe and, on average, about 10 percent better fuel economy.

It's hip to be undersquare with 's new FB horizontally opposed 4-cam 4-cylinder, replacing the EJ that dates back to 1989. Where the EJ was oversquare (i.e., bore larger than stroke), the FB's smaller-diameter cylinders (84 mm versus 92) allow for more compact combustion chambers, a boon for better combustion and lower emissions. The trick was to increase the stroke of the crankshaft (now 90 mm versus 75), yet not increase the width of the longitudinally mounted engine—lest it cease to fit into existing Subaru vehicles. The solution was swapping the camshafts' belt drive for chain drive (and consequently, bulkier pulleys for smaller sprockets). The valve angle was reduced too, from 41 degrees to 27—further shrinking the size of the combustion chamber. For lower friction, cam lobes now ride on roller tappets and cylinder-bore roundness was improved, allowing for lower-tension piston rings. The intake system has been totally reworked to include a plastic plenum/runners assembly and four butterfly valves (in addition to the single throttle body) to enhance cylinder filling or promote tumble when needed. The upshot? About the same power as before in both 2.0- and 2.5-liter iterations, but slightly more midrange torque, sweeter air out the tailpipe and, on average, about 10 percent better fuel economy.